A Neutral Point Voltage Balancing Control Method for Three-Level Three-Phase Four-Wire Inverters With Fewer Switching State Transitions

The four-leg three-level three-phase four-wire (TLTPFW) inverters combine the advantages of both the three-level topology and four-leg topology with low harmonic content, low voltage stress on the power devices, and high load carrying capacity. In this paper, a neutral-point (NP) voltage balancing control method with few switch-state transitions is proposed to completely eliminate low-frequency NP voltage oscillations while having the same number of switch-state transitions as a conventional carrier-based pulse width modulation (CBPWM). The proposed method uses discontinuous pulse width modulation (DPWM) in the region where the NP voltage balancing control capability of CBPWM is insufficient to achieve NP voltage balancing control in the whole region. In the DPWM method, the switching state of one leg remains unchanged during a single sampling period, the switching state of the other leg is switched twice, and the switching state of each of the remaining two legs is switched once, which ensures that the total number of switching transitions of the four legs is four during a single sampling period. Finally, the simulation and experimental studies of the proposed method are carried out, and the simulation and experimental results verify the correctness and effectiveness of the proposed method.